In a temperature regulating device or a combustion control device, when a motor, an igniter, a fuel feed value or a pump, or a heater is to be driven, the conduction or non-conduction of such load is controlled by a semiconductor switch control element or relay. If, in this case, a conduciton problem occurs in a switching element, the danger arises that the load is driven irrespective of the existence of a final control output signal. As a countermeasure therefor, in the analog control device according to the prior art, a first method has been practiced in which the load such as the relay is constructed into a special fail-safe circuit; a second method, in which a plurality of switching elements are connected in parallel; and a third method, in which the final stage circuit is so constructed that the load is made to work safely, even if a conduction problem occurs in the final stage, by the action of the remaining circuit. Specifically, the first method is disclosed in Japanese Patent Publications Nos. 54-12317 and 46-31905, and in Japanese Utility Model Publication No. 51-28944, and the third method is disclosed in Japanese Patent Publication No. 51-18654.
All of these methods have been practiced by making the circuit itself fail-safe or doubly safe. In the recent digital control device, however, to make the circuit fail-safe on the basis of such concept is accompanied by serious difficulties.
The present invention contemplates providing a novel check circuit for the conduction problem of a load control element while succeeding in overcoming such difficulty.